Question

Are the dried up river valleys seen on Mars, likely to be Millions, or Billions of years old?

The most common way to estimate the age of surface feature on Mars or other planets is by the numbers of impact craters superimposed on the topography. Only for the Earth and Moon do we have samples that can can be dated radiometrically. Cratering is a universal process in the solar system, however, and dating by crater density works pretty well when we have good photos from orbit, as we do for Mars. The channel networks in the highlands are the oldest water erosion features, an they are quite old, perhaps as much as 3-4 billion years. Next are the large outwash channels associated with massive flooding; these are typically 2-3 billion years old. Much younger are the small "gullies" discovered in recent high-resolution surveys. These gullies are common in scarps and crater walls. Many of these have no superimposed craters at all, and thus are no more than a few million years old, perhaps much less. Thus the major water erosion features are from a distant time, but there are smaller gullies that seem to be contemporary.

David Morrison
NAI Senior Scientist
November 12, 2008